Solid Natural Rubber Masterbatch Strengthens Asphalt Roads Across Three Rock Types

Asphalt concrete is the workhorse of modern road networks, and its performance hinges on two partners: the bituminous binder that glues the structure together and the mineral aggregate skeleton that carries the load. When traffic volumes and axle weights climb, this partnership comes under strain, and pavements begin to fail before their design life is reached. Rutting, fatigue cracking, and moisture-induced stripping are the familiar symptoms. A research team in Thailand, led by Apinun Buritatum and Suksun Horpibulsuk of Suranaree University of Technology, has now shown that a solid form of natural rubber called Masterbatch Natural Rubber, or MNR, can substantially toughen asphalt concrete across the three aggregates most used in the country: basalt, granite, and limestone. The work, published in Cleaner Engineering and Technology, was supported by the Rubber Authority of Thailand and aligns with a national strategy to raise domestic rubber consumption in public infrastructure.

The motivation for the study lies in a long-standing dilemma of pavement engineering. Synthetic polymer modifiers such as styrene-butadiene-styrene, ethylene vinyl acetate, and styrene-butadiene rubber undeniably improve binder rheology, forming three-dimensional networks that boost elasticity, temperature stability, and crack resistance. Yet these polymers are derived from petrochemicals, raising concerns about cost, recyclability, and sustainability. Natural rubber, a biopolymer of poly-cis-1,4-isoprene, offers a renewable alternative that has already demonstrated gains in penetration index, softening point, elastic recovery, and dynamic shear modulus when blended into asphalt. Thailand, Indonesia, and Malaysia, the world’s leading natural rubber producers, have actively promoted its use in road construction. The catch has been logistical: liquid latex forms such as concentrated latex and pre-vulcanized latex have limited shelf lives, are prone to coagulation, and demand specialized storage and handling that can hinder large-scale field deployment.

MNR was developed precisely to overcome those constraints. It is produced by concentrating raw natural rubber through centrifugation to a dry rubber content above 60 percent, stabilizing it with sodium dodecyl sulfate, antioxidants, and zinc oxide, then blending in carbon black as a reinforcing filler at 70 degrees Celsius. A two-stage vulcanization with sulfur, tetramethylthiuram disulfide, and zinc diethyldithiocarbamate follows at 140 and 160 degrees Celsius to promote cross-linking. The mixture is finally coagulated in 90 percent acetic acid, filtered, and vacuum-dried into a solid product that ships and stores like any other granular additive. In the laboratory, the solid MNR particles were dispersed into a 60/70 penetration-grade asphalt cement using controlled thermal stirring at 170 degrees Celsius and 180 revolutions per minute, with homogeneity confirmed by the absence of undissolved rubber fragments.

The researchers tested rubber-to-binder ratios ranging from 0 to 15 percent against three aggregates sourced from Thai quarries: basalt from Buriram Province and granite and limestone from Saraburi Province. All three met the Thailand Department of Highways specifications for wearing surface layers, but their mineralogy differs in ways that matter. Alkaline basalt and limestone generally promote stronger binder adhesion, whereas acidic granite tends to bond more weakly, a difference rooted in surface free energy, the thermodynamic measure of a material’s tendency to bond with or repel liquids. Mixtures were designed by the Marshall method to a target air void content of 4 percent, with specimens compacted under 75 blows per side and evaluated through indirect tensile strength, resilient modulus, fatigue life, and Hamburg wheel tracking tests.

The headline finding is a strikingly consistent optimum. Across every aggregate type and every mechanical test, performance peaked at a rubber-to-binder ratio of 3 percent. At that dosage, Marshall stability reached 19.8 kilonewtons for basalt mixtures, 17.6 for granite, and 12.5 for limestone, all comfortably above the 8.0 kilonewton specification minimum. Indirect tensile strength followed the same hierarchy, peaking at 587.5, 482.2, and 437.1 kilopascals respectively. The resilient modulus and fatigue life also crested at 3 percent, while rut depths after 10,000 wheel-tracking cycles in a 50-degree-Celsius water bath reached their minimum. Beyond the optimum, performance declined steadily, a consequence of excessively thick latex films disrupting stress transmission across the binder-aggregate interface and of dispersion problems within the asphalt matrix.

The mechanism behind the optimum is the formation of a cohesive latex-film network within the binder. Cross-linked and bridged rubber molecules reinforce elastic behavior, reduce flow susceptibility, and strengthen the adhesive interface between binder and aggregate. Notably, the 3 percent optimum coincided with peak elastic recovery of 50 percent in the modified binder, and elastic recovery showed strong linear correlations with both tensile strength and resilient modulus. This makes elastic recovery a practical screening indicator for mix design, allowing engineers to predict mixture performance from a simple binder test. The basalt mixtures displayed the steepest dependence on elastic recovery, suggesting that high-surface-energy aggregates amplify the benefit of the rubber network.

Quantifying the gains against unmodified control mixtures revealed how much aggregate character matters. Basalt mixtures improved by 12.0 percent in stability, 13.2 percent in tensile strength, 31.3 percent in resilient modulus, 32.2 percent in fatigue life, and 49.4 percent in rutting resistance. Limestone mixtures followed with gains of 7.9, 8.3, 25.9, 15.3, and 43.8 percent, while granite mixtures posted 6.0, 4.8, 17.3, 10.6, and 31.6 percent. Rutting told a subtler story: granite mixtures showed the lowest absolute rut depths, thanks to the angular texture and high interparticle friction of the stone, but basalt achieved the largest percentage improvement, implicating surface free energy as the key to how much a mixture benefits from modification. In a particularly encouraging result, the modified limestone mixture outperformed even the unmodified basalt mixture in rutting, showing that MNR can upgrade lower-quality aggregates.

To translate the laboratory data into design tools, the team developed a unified fatigue distress model linking fatigue life to initial tensile strain through a power-law relationship, the framework used in mechanistic-empirical pavement design. A strong linear correlation between indirect tensile strength and resilient modulus, with a coefficient of determination of 0.93, allows engineers to estimate mechanistic properties from routine static tests regardless of aggregate type. The analysis also revealed a clear division of labor: aggregate type sets the initial mechanical baseline, while the rubber-to-binder ratio governs the trajectory and magnitude of enhancement. Initial tensile strain reached its minimum at 3 percent and rose sharply at higher rubber contents, especially under the most severe stress levels of 300 and 350 kilopascals, confirming that over-modification softens the binder and undermines stiffness.

Compared with concentrated latex-modified asphalt at the same optimum dosage, the MNR mixtures showed slightly lower fatigue life, attributable to a lower resilient modulus, meaning the liquid latex route still holds a modest edge in pure fatigue performance. But the solid form eliminates sealed containers, controlled storage, and coagulation risks, simplifying transport and reducing costs for large or remote projects, advantages the authors note were not directly quantified in this study. The work remains laboratory-scale, without field trials, cost analysis, or life-cycle assessment, and the authors recommend scanning electron and atomic force microscopy to confirm the interfacial mechanisms they infer. Even so, the message is compelling: a modest 3 percent dose of solid natural rubber, matched with high-surface-energy aggregate, can meaningfully extend pavement life using a renewable crop that rubber-farming regions already grow in abundance.

Subject of Research: Natural rubber masterbatch modification of asphalt concrete with different aggregate types

Article Title: Masterbatch natural rubber improved mechanical performance of asphalt concretes with various natural aggregates

Article References: Buritatum, A., Horpibulsuk, S., Suddeepong, A., Yaowarat, T., Aiamsri, K., Hoy, M., Akkharawongwhatthana, K., Laomuad, A., & Chinkulkijniwat, A. (2026). Masterbatch natural rubber improved mechanical performance of asphalt concretes with various natural aggregates. Cleaner Engineering and Technology, 35, Article 101332. https://doi.org/10.1016/j.clet.2026.101332

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101332

Keywords: asphalt concrete, natural rubber, masterbatch, pavement engineering, binder-aggregate adhesion, fatigue life, rutting resistance, surface free energy, sustainable materials, polymer modification, Marshall stability, Thailand